1. Field of the Invention
The present invention relates to techniques for estimating properties of an earth formation penetrated by a borehole and, in particular, to estimating saturation of three fluid phases in the earth formation.
2. Description of the Related Art
In commercial production of hydrocarbons, the hydrocarbons are extracted from reservoirs in earth formations. The reservoirs of hydrocarbons are generally contained in the pores of rock in the earth formations. The hydrocarbons can include oil and gas. In general, water in addition to the hydrocarbons may also be present in the pores. Hence, it is of considerable value to know where in the earth formation the types of hydrocarbons and associated concentrations are for efficient use of production resources.
Well logging is a technique used to perform measurements of an earth formation. In well logging, a logging instrument or tool is conveyed through a borehole penetrating the earth formation. The logging tool is configured to perform the measurements from within the borehole. Measurements are typically performed at various depths in the borehole. The measurements are associated with the depth at which they were performed to produce a log. Data from some of the logs may be used to determine relative amounts (i.e., saturation) of oil, gas or water in the pores of the earth formation.
Unfortunately, some of those logs have to be performed in an uncased borehole and typically require several passes through the borehole with different type of logging tools in order to get enough data to determine the saturation of a formation fluid. Even more data may be required to determine three-phase saturation, i.e., the saturation of oil, gas and water in the pores. If measurements to obtain the extra data have to be performed in an uncased borehole, then the risk of borehole damage can increase due to the delay in well completion.
Therefore, what are needed are techniques to efficiently determine a saturation of a fluid in an earth formation. Preferably, the techniques can be used in a cased borehole penetrating the earth formation and to determine a saturation of three fluids in the earth formation.